Device for electrohydraulic forming of sheet metal

ABSTRACT

A device for electrohydraulic forming of sheet metal includes an upper die part having a die plate, and a lower die part on which an electrode frame is disposed. The frame accommodates at least two electrodes, and a metal sheet can be clamped in place between the die plate and the electrode frame in fluid-tight manner. At least one potential-free bridge element is disposed between at least two electrodes, within the electrode frame.

CROSS REFERENCE TO RELATED APPLICATIONS

Applicant claims priority under 35 U.S.C. §119 of European ApplicationNo. 09011365 filed on Sep. 4, 2009.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a device for electrohydraulic forming of sheetmetal including an upper die part having a die plate and a lower diepart on which an electrode frame is disposed, which frame accommodatesat least two electrodes whereby a metal sheet can be clamped in placebetween die plate and electrode frame.

2. The Prior Art

Various methods, tailored to the product, in each instance, are used forforming metal sheets. In the case of conventional pressing, the metalsheet to be formed is pressed against the die plate that has the desiredcontour, by way of a punch. In the case of hydroforming pressing,however, the sheet metal is pressed against the die place using a fluid,preferably water, that has been put under pressure. In the hydroformingmethod, forming pressures of 2000 to 4000 Bar are produced. Depending onthe size of the area to be formed, correspondingly great forces to holdthe hydroforming press closed are therefore required, because theforming pressure that is applied is present on the entire area to beformed at the same time. Because the hydroforming press can build therequired pressure up only slowly, long cycle times result, accordingly,during forming. In this connection, as in all the other conventionalmethods, the shape changes of the sheet metal occur because the rows ofatoms within the individual crystals are displaced relative to oneanother when a certain limit tension is exceeded, and cohesion betweenthe rows of atoms on the nearest subsequent lattice position takesplace.

Improved material properties are achieved with high-speed formingmethods, for example the electrohydraulic method. In this connection,the die on the side of the metal sheet to be formed, opposite the dieplate, is completely filled up with a transfer medium, preferably water.In the space filled with water, underwater electrodes are disposed, byway of which electrical energy stored in capacitors is suddenlydischarged. As a result of the capacitor underwater discharge, the wateris explosively accelerated against the metal sheet, which is hereby laidagainst the die plate at high speed. In contrast to the conventionalhydroforming method, in the case of the high-speed forming that takesplace here, so-called “twin formation” occurs. During such twinformation, permanent deformation takes place as the result of folding ofthe lattice from one position into the other. This twin formation leadsto better material properties after forming, in other words of thefinished product, for one thing; for another, clearly better materialproperties are also present during forming. Materials that are difficultto form, such as various titanium or aluminum alloys, for example, canbe formed only under high-speed conditions.

This method has the disadvantage that the electrical discharge of anelectrode pair brings about an essentially point-shaped energydischarge, and this discharge results in a hydraulic field of effecthaving a diameter of maximally 200 mm. To increase the field of effect,multiple electrode pairs are therefore necessary.

In this connection, it must first of all be taken into considerationthat a long-lasting high-power switch that suddenly supplies theelectrodes with the required electrical energy should be present in thesystem only once, for cost reasons. Thus, tight restrictions are placedon the circuitry of the electrode pairs according to the present stateof the art: If multiple electrode pairs are switched in parallel by wayof a high-power switch, no reproducible results are achieved, becausethe spark gap that ignites first already withdraws a major portion ofthe available energy from the other spark gaps (electrode pairs).

Because of complex ionization conditions in the transfer medium, thespark gaps switched in parallel cannot all ignite at the same time. If,on the other hand, multiple electrode pairs are switched in series, thenthe problem of simultaneous ignition is eliminated, but the possiblenumber of spark gaps that can be switched in series is limited by therequired ignition voltage.

SUMMARY OF THE INVENTION

It is an object of the present invention to remedy this situation and toprovide a device for electrohydraulic forming of sheet metal that allowsefficient forming of sheet metal even with larger forming areas, forseries production. According to the invention, this task is accomplishedby disposing at least one potential-free bridge element between at leasttwo electrodes within the electrode frame.

With the invention, a device for electrohydraulic forming of sheet metalis created, which allows efficient forming of sheet metal even withlarger forming areas. By placing at least one potential-free bridgeelement between two electrodes, simultaneous production of multiplespark gaps at which the electrical energy is converted to hydraulicenergy, disposed spaced apart from one another, takes place during thecapacitor discharge, thereby allowing forming over a large area. Thebridge element is not connected electrically from the outside, butrather serves as a bridge in the power circuit, depending on itsposition. The electrical connection of the electrodes is selected insuch a way that the bridge element produces a bridge from plus to minusin every position. In the case of the present serial circuit of thespark gaps, the great plasma current that is responsible for theconversion of the electrical energy into hydraulic energy can only flowonce all the spark gaps that are switched one behind the other haveignited.

The at least one potential-free bridge element can be disposed in afixed position, so as to rotate, or so as to be displaceable within theelectrode frame. The rotating and/or displaceable placement of apotential-free bridge element makes possible the use of a bridge elementfor use in different electrode pairs—switched one behind the other interms of time. Because the bridge element can be positioned, itfurthermore replaces a high-power switch that would otherwise berequired multiple times.

Preferably, a potential-free bridge element is connected with a drive byway of which it can be moved. In this way, the assignment of a bridgeelement to an electrode pair, in each instance, can be controlled fromthe outside.

In a further development of the invention, a control device is disposed,which is connected with the at least one drive and is set up in such amanner that different spark gaps can be activated, at predeterminedtimes, changing the position of at least one potential-free bridgeelement. In this way, an automatic sequence of discharges at differentlocations is made possible, thereby making it possible to achievestep-by-step forming of a metal sheet, even one having a larger formingarea.

In an embodiment of the invention, a line for supplying fluid isprovided in the lower die part. In this way, additional transfer media(for example water) can be supplied for balancing out the volume changeof the chamber filled with the transfer medium that goes along with theforming process.

In a further development of the invention, a second die plate isdisposed in the lower die part, whereby a second metal sheet can beclamped in place between electrode frame and lower die part, influid-tight manner. In this way, simultaneous forming of two metalsheets during a discharge sequence is made possible, thereby clearlyincreasing the productivity of the device.

In an embodiment of the invention, at least two electrodes areconfigured as electrodes that can be advanced to compensate wear. Inthis way, it is possible to compensate for the electrodes being burntaway and, at the same time, also for the bridge elements being burntaway. Preferably—additionally or alternatively—at least one bridgeelement is configured as an element that can be advanced to compensatewear.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and features of the invention will become apparent fromthe following detailed description considered in connection with theaccompanying drawings. It is to be understood, however, that thedrawings are designed as an illustration only and not as a definition ofthe limits of the invention.

In the drawings,

FIG. 1 is a schematic representation of a device for electrohydraulicforming of sheet metal;

FIG. 2 is a schematic representation of the electrode frame of thedevice of FIG. 1, having a rotating bridge element;

FIG. 3 is a schematic representation of an electrode frame in anotherembodiment, having a displaceable bridge element;

FIG. 4 is a schematic representation of an electrode frame in a thirdembodiment, having multiple rotating bridge elements;

FIG. 5 is a schematic representation of an electrode frame in a fourthembodiment, having a combination of multiple fixed and rotating bridgeelements; and

FIG. 6 is a schematic representation of a device for electrohydraulicforming of sheet metal, having a second die plate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Turning now in detail to the drawings, FIG. 1 shows an exemplaryembodiment of a device for electrohydraulic forming of sheet metalessentially made up of an upper die part 1 and a lower die part 2,between which an electrode frame 3 is disposed. A metal sheet 4 can beclamped in place between electrode frame 3 and upper die part 1, influid-tight manner.

A die plate 11 is placed into upper die part 1, which plate has thecontour to be shaped into metal sheet 4. Furthermore, a line 10 forevacuating the chamber formed by metal sheet 4 laid in and die plate 11is provided in upper die part 1.

Lower die part 2 has reflectors 21 configured essentially parabolicallyor elliptically, which are disposed to lie opposite die plate 11 ofupper die part 1. Reflectors 21 serve to reflect discharge energy thatimpacts them, in the direction of die plate 11.

An electrode frame 3 is disposed on lower die part 2. In the exemplaryembodiment according to FIG. 2, electrode frame 3 is configured to beessentially square. Electrodes 31, 32 are disposed centered on the sidesof electrode frame 3, in each instance, whereby a negative electrode 32is disposed opposite a positive electrode 31, in each instance.Electrodes 31, 32 are surrounded by an insulator 30, in each instance. Apotential-free bridge element 33 is disposed in electrode frame 3 so asto rotate, centered between electrodes 31, 32, in such a manner that itcan be aligned with an electrode pair 31, 32, in each instance. Here,four spark gaps result, which lie on a diameter that corresponds to thelength of the bridge element.

In the course of the electrohydraulic method, a metal sheet 4 to beformed is placed onto electrode frame 3, and upper die part 1 and lowerdie part 2 are closed using hydraulic or mechanical force, so that metalsheet 4 is clamped in place between electrode frame 3 and upper die part1, in fluid-tight manner. The air from the chamber 12 formed by dieplate 11 and metal sheet 4 is evacuated by way of line 10. The fillingspace 22 that results between lower die part 2 and metal sheet 4 iscompletely filled with transfer medium, for example water. Subsequently,any desired number of discharges are carried out, one after the other,whereby metal sheet 4 is further deformed during every process step,because the water accelerated by the capacitor underwater dischargestill has sufficient forming energy even at a distance of a fewcentimeters. After every process step, i.e. after every dischargeprocess, the vacuum that has formed in filling space 22 as the result ofthe forming can be filled up with water by way of a line passing theelectrode frame 3.

The particular feature of this pulse method is that the required formingenergy does not necessarily have to be produced with one pulse. Themaximal pressure that occurs, i.e. the maximal speed of the acceleratedwater, is the same per pulse when the parameters are the same, but theforming might not be carried out completely, because the time per pulseis not sufficient. Thus, forming is continued accordingly when the nextpulse is carried out.

It is advantageous for the design and the useful lifetime of the devicecomponents, in this connection, that forming does not have to take placewith an energy pulse at very high energy, but rather can take place bymeans of multiple energy pulses having correspondingly lower energy.These energy pulses can take place distributed over the area of thespark gaps formed by the bridge elements 33, thereby increasing theeffective forming area.

In the exemplary embodiment according to FIG. 3, two electrode pairs 31,32 are disposed parallel to one another, along with a potential-freebridge element 33 that is disposed to be displaceable between the twoelectrode pairs 31, 32. Here, four spark gaps result, which lie in thecorner points of a rectangle, whereby the one side of the rectangle isdescribed by the length of the bridge element, and the second side bythe distance between electrodes 31 and 32, respectively, on one side ofelectrode frame 3.

The significant advantage of the displaceable or rotating bridge elementis that only two of the four spark gaps ignite, in each instance,because only these spark gaps lie in a power circuit capable ofigniting. This feature is particularly advantageous because a certainminimum voltage must be applied for each individual spark gap, in orderto achieve reproducible ignition behavior and thus a high level ofconversion of electrical to hydraulic energy.

The required minimum ignition voltage for a spark gap is essentiallydependent on the diameter and shape of electrode 31, 32 and on thedistance between the electrodes, on the electrical conductivity of thetransfer medium (for example water), and on the pressure level thatexists in the transfer medium just before moment of ignition. A plasmaarc occurs at every ignition of the spark gap, and leads to acorresponding burn-off rate at the electrodes. Under the aspect of longlife for series production, electrode diameters <10 mm are unsuitable atgreater energy values, as they are required for sheet steel having amaterial thickness of >0.4 mm. In order to allow efficient spread of thehydraulic energy that occurs in the form of points between electrodes31, 32, on the plasma channel that forms, a minimum ignition voltage ofapproximately 10 kV per spark gap is required, for example, when usingwater as a transfer medium.

In the exemplary embodiment according to FIG. 4, two electrodes 31, 32having the same poles are disposed next to one another, lying next toone another on each side of the electrode frame 3. Within electrodeframe 3, four potential-free bridge elements 33 are disposed so as torotate, in such a manner that an electrode pair can be bridged by way oftwo potential-free bridge elements 33 that are disposed one behind theother, in each instance, whereby three spark gaps are formed, in eachinstance. In this manner, twelve different spark gaps are formed, whichcover a correspondingly large forming region.

In the exemplary embodiment according to FIG. 5, three pairs ofelectrodes 31, 32 and three bridge elements 33 that are fixed in placeand two that are disposed so as to rotate are disposed. In thisarrangement, three positions having two, three, or four spark gaps canbe covered, one after the other. The embodiment variants are any thatare desired; for example, rotating, displaceable, and fixed bridgeelements can also be combined. In this way, in total, the requirement ofcovering a forming region that is as large as desired is fulfilled.

For use of the electrohydraulic method in mass production, automaticadjusting of the electrode rod to replace wear can take place toincrease the useful lifetime of electrodes 31, 32, which burn down, and,at the same time, burning down of a displaceable or also a rotatingbridge element can also be compensated. After an extended period of use,the point of the spark flashover, in other words the spark gap itself,is displaced as a result. This displacement can be compensated in thatbridge element 33 itself is replaced once a shift, for example, or oncea day.

Another essential aspect of the invention consists in that in the caseof the design of the electrode array according to the invention,fundamentally only one switch is required, even if twelve spark gaps aredisposed, for example, of which only a few specific ones are ignited,depending on the position of bridge elements 33. The entire energystored in the capacitors is thus divided up only among a few electrodesand bridge elements, respectively. In the total power circuit, bridgeelements 33 thus replace the switches, which must otherwise be used inmultiples. Such high-power switches are the most cost-intensivecomponent of the entire electrohydraulic method, because here, highvoltages, corresponding to the required ignition voltage, and extremelyhigh currents in the range between 10 and 300 kA, must be switched veryrapidly (in the μs range). Such a switch is either very expensive whendesigned for a longer useful lifetime, or it demonstratescorrespondingly great wear in a less expensive version. In the case ofthe object of the invention, the positions of the activated spark gapsare automatically determined by the positions of bridge elements 33. Inthis way, the production costs of the entire system can be significantlyreduced.

In the exemplary embodiment according to FIG. 6, a second die plate 23is formed into lower die part 2 in place of a reflector 21. In thisconnection, electrode frame 3 is attached independent of upper die part1 and lower die part 2, so that a metal sheet can be clamped in placebetween upper die part 1 and electrode frame 3 and between electrodeframe 3 and lower die part 2, in each instance. The chambers 12, 24formed between the metal sheets 4 and the die plates 11, 23 can beevacuated by way of lines 10, 20 provided in upper die part 1 and lowerdie part 2, in each instance. By means of the capacitor underwaterdischarge that took place by way of the underwater electrodes 31, 32 andthe bridge element 33, the accelerated water is accelerated against themetal sheets 4 that are disposed parallel to one another, therebypressing metal sheets 4 against die plates 11, 23. With thisarrangement, simultaneous deformation of two metal sheets in one formingprocess is possible.

Although only a few embodiments of the present invention have been shownand described, it is to be understood that many changes andmodifications may be made thereunto without departing from the spiritand scope of the invention.

What is claimed is:
 1. A device for electrohydraulic forming of sheetmetal comprising: (a) an upper die part having a die plate; (b) a lowerdie part; (c) an electrode frame disposed on the lower die part so thata metal sheet can be clamped in place between the die plate and theelectrode frame; (d) first and second electrodes disposed within theelectrode frame; and (e) at least one potential-free bridge elementdisposed between said first and second electrodes.
 2. The deviceaccording to claim 1, wherein the at least one potential-free bridgeelement is disposed in a fixed position.
 3. The device according toclaim 1, wherein the at least one potential-free bridge element isdisposed so as to rotate.
 4. The device according to claim 1, whereinthe at least one potential-free bridge element is disposed in adisplaceable manner.
 5. The device according to claim 3, wherein the atleast one potential-free bridge element is connected with at least onedrive for moving the at least one potential-free bridge element.
 6. Thedevice according to claim 5, further comprising a control deviceconnected with the at least one drive and configured so that differentspark gaps can be activated, at predetermined times, changing theposition of the at least one potential-free bridge element.
 7. Thedevice according to claim 1, further comprising a line for supplyingfluid provided in the electrode frame or in the lower die part.
 8. Thedevice according to claim 1, further comprising a second die platedisposed in the lower die part so that a second metal sheet can beclamped in place between the electrode frame and the lower die part influid-tight manner.
 9. The device according to claim 1, wherein at leasttwo electrodes can be adjusted to compensate wear.
 10. The deviceaccording to claim 1, wherein the at least one potential-free bridgeelement can be adjusted to compensate wear.